1. Field of the Invention
The present invention relates to optical fiber sensing systems and, more specifically, to a method and system for multiplexing a network of parallel fiber Bragg grating (FBG) sensor-fibers to a single acquisition channel of a closed Michelson interferometer system via a fiber splitter by distinguishing each branch of fiber sensors in the spatial domain.
2. Description of the Background
There are many diverse engineering applications where determining or monitoring the shape of an article or structure is of paramount importance. The science of determining changes to the material and/or geometric properties of a structure is referred to as Structural Health Monitoring (SHM). Generally, SHM involves the observation of a structure over time using periodically sampled measurements from an array of sensors, and the analysis of these measurements to determine the current state of structural health. There are many different sensors and sensing networks for accomplishing this, but many have inherent limitations that render them unsuitable for certain applications. In the field of aeronautics, for example, traditional structural health-monitoring of aircraft wings involves the use of photogrammetry. In photogrammetry, strategic portions of the aircraft are marked beforehand and a baseline photograph is taken and calibrated to determine the initial displacement. When the wing of the aircraft is under deflection either through structural testing or at flight, wing-deflection bending is monitored by comparing pre-deflection and post-deflection photographs. However, this technique requires a clear line of sight, and oftentimes the use of any direct line-of-sight monitoring is either impossible or impractical. The same rationale holds for bridges, other concrete structures, and most any solid structure where there is no line of sight through the structure itself.
Other monitoring techniques include electrically-wired networks of strain sensors, temperature sensors, accelerometers, or the like. However, these involve complex wiring layouts which are costly and impractical, and indeed the wires tend to corrode and break with age. Electronics-based sensors are also perpetrators of electromagnetic interference (EMI) which is an undesirable byproduct in a supposedly non-invasive sensing system.
Another technique that is rapidly gaining in popularity involves fiber optic sensing networks. See, e.g., Tennyson, “Monitoring Bridge Structures Using Long Gage-Length Fiber Optic Sensors”. Caltrans Bridge Research Conference (2005). Optical fiber sensors typically involve a light propagating beam which travels along an optical fiber network. Within each fiber the light is modulated as a function of strain, temperature, bending or other physical or chemical stimuli. The modulation can be analyzed in either reflection or transmission to determine the characteristic of interest. Optical fiber sensors (OFS) have many distinct advantages including immunity to electromagnetic interference, long lifetime, lightweight, small size, low cost, high sensitivity, etc. Serially multiplexed or branched OFS networks are especially suitable for SHM of large and/or distributed structures which usually need a lot of measurement points.
Optical fiber sensors (OFS) are typically composed of numerous optical fibers and numerous Fiber Bragg gratings (FBGs) periodically-spaced along the length of each fiber. Each FBG creates a periodic variation of the optical refractive index in the core of its associated optical fiber, and when coupled to an interferometer it becomes possible to detect strain individually through change in its resonant wavelength (i.e., the wavelength at which each grating has its maximum reflectance).
With optical frequency domain reflectometry (OFDR) all the FBGs are supplied with laser light having the same central wavelength, and their positions along the fiber are detected by measuring the beat frequency of any individual grating's reflection with the reflection from a reference arm of the interferometer having a known length. As the laser wavelength is scanned (where the scanning wavelength is inversely proportion to the its frequency), a small but measurable difference in the time for light to travel from the reference arm and from each grating is observed, causing variation in the interference pattern between the signal from the reference arm and the sensor arm. The frequency components of the interference pattern are linearly related to the distance along the fiber of the grating reflector. To illustrate, FIG. 1 is a perspective diagram illustrating an exemplary OFDR system without multiplexing, in which an OFDR reflectometer is optically coupled to one sensing fiber to interrogate all locations of interest. As seen at (A) sensor acquisition occurs when one continuous length of fiber is interrogated under OFDR resulting in an interference pattern presented as an “interferogram” (a graph of the variation of the output signal in the wavelength domain which encodes all beat frequencies of the sensor). As seen at (B), to obtain a sensor data that's of interest, a Fast Fourier Transform (FFT) signal-processing method is applied to convert the wavelength/frequency domain to the spatial/time domain so that the beating frequency for each FBG encoded within the interferogram can be uniquely identified via distance. The Fourier transform (FFT) of the reflected interferogram gives the Fourier impulse as a function of distance along the fiber. Thus the reflectivity of each grating can be measured. As seen in the enlarged inset of (B), to transform the wavelength (and strain) information from a particular sensor from the spatial domain back to the wavelength domain, a windowing function is first taken to lock-in the desired grating in spatial domain. Then, as seen at (C) an inverse Fourier Transform (iFFT) is taken to identify the resonant wavelength of the particular sensor. Since strain information is linearly proportional to the wavelength shift relative to the initial resonant wavelength of the fiber grating, the strain information can be obtained from the corresponding wavelength shift information.
The foregoing approach requires an un-interrupted section of fiber and is straightforward in a single-arm OFS network. However, it limits the ability to monitor different branched sections. Should any breakage occur throughout the continuous fiber the entire system is susceptible to loss of data after the breakage.
Monitoring a multiplexed or branched OFS network involving multiple fiber sensing arms each having multiple FBGs along their length is significantly more complicated. Thus, by way of example, in order to monitor three parallel fiber sections ten feet apart simultaneously, there are currently three solutions: 1) process each fiber section separately (which essentially requires multiple interferometers and computational complexity to interpret the three results); 2) join the three sections together (which adds unnecessary optical fiber increasing system cost and increases the risk of fiber breaking; or 3) multiplexing three segments into one acquisition channel.
Processing each fiber section separately is exemplified in B. Childers et al., “Use Of 3000 Bragg Grating Strain Sensors Distributed On Four Eight-Meter Optical Fibers During Static Load Tests Of A Composite Structure,” Proc. of SPIE, 4332, 133-142 (2001). This article shows a four channel optical network in which data from four fiber branches was acquired simultaneously and stored in four arrays, and the raw data was processed using four computer A/D channels to yield a single strain value per FBG per laser scan.
Joining fiber arms is exemplified by U.S. Pat. No. 4,770,535 to Kim et al. issued Sep. 13, 1988 (Stanford), which shows an array of fiber-optic sensors organized in a ladder configuration which is applicable only to transmission fiber sensors since it utilizes a Mach-Zehnder setup to produce interference between each ladder. Each Mach-Zehnder interferometer acts as one sensor, which is not applicable to FBG-based sensor deployment.
Multiplexing techniques can greatly simplify the optical, electronic and computational complexity. However, it becomes necessary to distinguish each FBG along each sensor-fiber, and also to distinguish each of the parallel FBG sensor-fibers. This is difficult in the context of a Michelson interferometer system that multiplexes a network of parallel FBG sensor-fibers to a single acquisition channel. Such is a “closed” system in which a laser source is swept, and the FBGs operate in reflectance mode, reflecting light. Combining multiple FBG sensor arms results in an equal factor loss in power because optical splitter/coupler inherently loses light. In closed systems the multiplex approach can result in a “light-starved” sensor.
Multiplexing is known in open systems that rely on other interferometer types and/or other types of sensor gratings that work in transmission mode, such as long period gratings (LPGs). For example, United States Patent Application 20110273719 by Froggatt; Mark E. (Luna Innovations) discloses Optical Imaging For Optical Device Inspection using OFDR. At para [0050] a generic suggestion is made that it is possible to time-delay multiplex multiple detection fibers, creating in each detector fiber a unique delay so that each fiber has a “slot” in the total scan length in which its impulse response signal resides. However, Froggatt '3719 is confined to a fiber-optic imaging system that works in transmission mode for collecting scattered light in multiple fibers at multiple locations. Froggatt's FIG. 10 shows an example where scatter is measured from a Bragg grating using a separate collection fiber 36 in a DUT, and the time-delayed multiplexing allows the capture of more light. This approach is unsatisfactory for a system using FBGs in reflectance mode because optical combiners inherently lose light. Indeed, Froggatt suggests “Although such a loss is significant, the time delay multiplex approach may be useful in applications where the system is not “light-starved.” Multiplexing a network of parallel fiber Bragg grating (FBG) sensor-fibers to a single acquisition channel of a closed Michelson interferometer system via a fiber splitter inherently results in a system that is “light-starved” and compels a different approach.
Similarly, Jiang M, Chen D, He S, “Multiplexing Scheme of Long-Period Grating Sensors Based on a Modified Optical Frequency Domain Reflectometry,” IEEE PHOTONICS TECHNOLOGY LETTERS, 20(21-24), 1962-1964, November-December (2008) shows a multiplexed OFDR network with presetting of different values for the optical path differences between the measuring arms and a static reference arm. The Jiang et al technology is confined to a multiplexing scheme using long-period grating sensors (LPGs) for optical frequency domain reflectrometry (OFDR) in a Mach-Zehnder interferometer.
What is needed is an alternative approach that multiplexes different sections of FBG-sensing fiber to a single acquisition channel of a Michelson interferometer in a branched-fiber network of FBGs in reflectance mode. Instead of one un-interrupted section of fiber sensors, multiple parallel fibers would be coupled together at a fiber splitter. A system and method with such features would have great utility in traditional SHM systems for most any engineering structures, and would find ready application in SHM as well as non-traditional shape sensing applications such as medical tools (e.g., flexible endoscopes and other minimally invasive surgical instruments) or other systems for monitoring and inspection.